ELEMENTS OF WOODWORK
ELEMENTS OF WOODWORK
Growth of Wood
1. Kinds of trees used for lumber.—(A.) The classification of trees here considered is based upon the method by which the trunk, or stem, of a tree is formed. The termexogenousis applied to outside growers, around which a layer of wood grows each year, and from which is cut the lumber of commerce. As the wood-worker is interested mainly in trees which grow by this method, we will do no more than mention theendogenous, or inside-growing, trees or plants of the nature of palm trees, cornstalks, etc., in which the woody fiber is formed upon the inside of the stem.
(B.) The new wood formed each year upon exogenous trees is known as theannual layer, orring; the separate layers being more prominent in open-grained woods, such as oak, ash, and chestnut, than in close-grained woods, such as maple, cherry, poplar, and birch. It is the difference in the character and structure of these layers which makes some woods hard and others soft, some with open and others with close grain, and which also, with the coloring matter peculiar to each kind of wood, causes its individuality and adaptability to certain uses.(The color and odor of wood are caused by chemical combinations, and are not part of the substance of the wood.)
Fig. 1.—Section of Yellow Pine.a, soft, cellular grain;b, hard, compactgrain.
Fig. 1.—Section of Yellow Pine.
a, soft, cellular grain;b, hard, compactgrain.
Each of these annual layers is composed of two parts, the formation being shown in Fig. 1, in which the grain of yellow pine is depicted. The soft, cellular, or open, grain,a, is formed as the sap moves upward in the spring, and the hard, compact grain,b, is formed later in the year. In soft woods the open grain predominates, while in hard woods the compact grain is more in evidence.
The age of a tree may be determined by counting these annual rings upon the stump, though a drouth during the growing season may have at some time so affected its growth as to make some layers indistinct, rendering it impossible to be absolutely sure of the count. In a young tree the annual layers are thicker than when the tree becomes more mature.
(C.) The different kinds of timber which the carpenter uses are cut fromdeciduous, or broad-leaf, trees, and fromconiferous, or needle-leaf, trees. This classification of trees is based upon the difference in the forms of their leaves, the former kind furnishing the ash, oak, walnut, beech, birch, and other woods that are hard to work, as well as poplar or white wood, linn or bass wood, and others whichare called hard woods, not in the sense that they are hard to work, but because their method of growth is the same.
Theconiferousorevergreentrees furnish the cedars, pines, hemlocks, spruces, firs, redwood, tamarack, cypress, and a few other woods of the same nature. These woods have a resin which is always present, no matter how old or dry the wood may be, which explains their superior weather-resisting qualities.
2. The formation of wood.—(A.) In the spring the sap begins to flow upward, mainly through the open cells of the cambium (the new growth in the stem, by which the diameter of the tree is increased), and to some extent through the sapwood. As it comes in contact with the air by means of the leaves and the green shoots of the tree, (B.) it gives off water and oxygen, and absorbs carbonic acid gas from the air, (C.) which is formed into starch, sugar, oil, etc.; this in turn becomes part of the sapwood, and forms the compact part of the annual ring, completing the layer for that year.
(D.) The wood is formed by a deposit of matter from the sap, which gradually thickens the cell walls until the cells are filled, when that layer becomes a part of the heartwood, or that part of the tree which is inclosed within the annual layers of sapwood.
(E.) Both the fibers and the cells of the sapwood are filled with water or sap, which may be removed by seasoning, but the sapwood of most trees used for building purposes is not as good lumber as the heartwood, as it is always susceptible to moisture. The exceptions to this will be mentioned elsewhere.
(F.) In all unseasoned lumber from 20 to 60 per cent ofits weight is moisture, which must be evaporated before the lumber has its highest commercial value. This may be done by weather drying or by artificial means, the lumber being treated in a specially constructed kiln.
3. Tissues.—(A.) A tree trunk is composed of four different tissues, viz.:
1. Thepith(Fig. 2,a), a cellular tissue: this is worthless and its presence in lumber is considered a defect.
2. Thewood, which includes the heartwood (b), the sapwood (c), and the medullary rays (g). From this part of the tree the commercially valuable lumber is taken.
3. Thecambium(d), which is a thin layer between the sapwood (c) and the bast (e).
4. Thebark, which includes both the bast (e) and the outer bark (f). The bark of most trees is worthless, but that of chestnut, oak, hemlock, and other trees which are rich in tannic acid is used in large quantities by tanneries.
(B.) The heartwood (b), orduramen, of most trees is the part generally used by carpenters upon the best work. It is firm, compact, and of the color and qualities characteristic of the wood.
(C.) The sapwood (c), oralburnum, is generally light-colored, and in most building woods its presence is considered a defect, though not in hickory, ash, maple, or yellow pine, and a few other woods; in fact, in these woods it is often preferred to the heartwood for many kinds of work on account of its color. The alburnum is filled with the active elements of the sap, which are deposited as the sap passes through the trees, and in time becomes part of the duramen, or heartwood. The time requiredfor sapwood to attain maturity ranges from thirty to one hundred years, according to its kind and age.
Fig. 2.—Section of Oak Tree Trunk.a, pith;b, heartwood, or duramen;c, sapwood, or alburnum;d, cambium;e, bast, or inner bark;f, outer bark, or corky layer;g, medullary rays, or silver grain;po, plain oak;qo, quartered oak.
Fig. 2.—Section of Oak Tree Trunk.
a, pith;b, heartwood, or duramen;c, sapwood, or alburnum;d, cambium;e, bast, or inner bark;f, outer bark, or corky layer;g, medullary rays, or silver grain;po, plain oak;qo, quartered oak.
(D.) Outside of the sapwood (Fig. 2,c) is the cambium (d), which furnishes the substance upon which the life of the tree depends. Here, nourished by the richest sap, new cells are formed, which become either sapwood or a part of the bast. (E.) At (e) is shown the bast or inner bark, which is composed of a woody fiber combined with a tissue of cells. This is elastic, which allows it to expand as the wood grows beneath it. Outside of the bast is the bark (f), or outer covering, which is of a corky nature, and protects the delicate vital parts of the tree.
4. The medullary rays.—Every tree hasmedullary rays(Fig. 2,g), usually spoken of by wood-workers as the “silver streak,” or “silver grain.” These rays connect the center of the tree with the outside, and are more prominent in such woods as oak, beech, and sycamore than in other woods. It is to take advantage of the beauty which these medullary rays impart that so much quarter-sawed lumber is used, though lumber sawed in this way is preferred for other reasons, which will be discussed later.
In many woods these rays are so small as to be invisible to the naked eye, as in pine, for instance, which has fifteen thousand to the square inch. Aside from adding much to the beauty of the lumber, they also give strength; if lumber is dried out too rapidly by artificial heat, it is apt tocheck, or crack, upon the line of the medullary rays.
Fig. 3.—Defects in Lumber.a, wind shakes or cup shakes;b, heart shakes;c, star shakes;d, branch broken off, showing the method by which the annual layers gradually cover broken branches;e, hard knot.
Fig. 3.—Defects in Lumber.
a, wind shakes or cup shakes;b, heart shakes;c, star shakes;d, branch broken off, showing the method by which the annual layers gradually cover broken branches;e, hard knot.
5. The grain in trees.—In open or exposed situations trees usually grow more or less gnarled or crooked, whichtends to improve the grain. The strength of lumber cut from trees grown as described is impaired, however, as the wood is more cross-grained than that from trees which grow in the heart of a forest, with practically the same conditions on all sides. As forest trees are continually reaching up for the sunlight, they grow taller andstraighter, which makes it possible for straighter-grained lumber to be cut from them, and as the branches are at the top, where they receive the sunlight and air, there are few cross-grained places.
When the annual rings are large, the grain is said to becoarse, and if the rings are fine, the termfine-grainedis used to describe it. When the direction of the fibers is nearly parallel with the sides and the edges of the board, it is said to bestraight-grained; when the lumber is taken from a crooked tree, it is said to becross-grained, as the grain follows the shape of the log, while the board is sawed straight. Cross-grained lumber is the handsomer; in this the fibers, being at different angles with the surface of the board, form a variety of figures, which add much to the beauty of the wood.
6. Defects found in lumber.—(A.) Some of the most common defects found in lumber arewind shakes, orcup shakes(Fig. 3,a), which are cracks following the line of the porous part of the annual rings. These are caused by the action of severe winds. (B.)Heart shakes(Fig. 3,b) are cracks radiating from the center of the tree, and may be found in any kind of wood, as they are the result of deficient nutrition, or loss of vitality. (C.)Star shakes(Fig. 3,c) are caused by the shrinkage of the tree upon the outside, which is the result of a long dry spell of intense cold, or of the deficient action of the sap. Star shakes differ from heart shakes in being larger upon the outside of the tree; the heart shakes are larger at the center.
Shakes of all kinds are common defects and sometimes are so numerous as to make the log worthless.
(D.)Discolorationsare caused by decay which has at some time gained a foothold, but which the tree was sufficiently vigorous to overcome; or they may be due to imperfect or insufficient nutrition, which generally results in the entire tree being affected instead of small places upon the tree.
(E.) Timber grown in a damp, marshy locality is generally of a poorer quality than that grown upon higher ground, as more water is taken up by the roots than can be well assimilated, which prevents the formation of healthy compact wood. Some woods are adapted to such soil, the poplar or whitewood, willow, basswood, buckeye, and cypress being denizens of moist or swampy places.
Trees growing where they are exposed to winds from one direction are apt to assume a spiral growth, which renders the timber almost worthless, as it is weak, and twists badly in drying.
(F.) Trees which have lumps appearing like growths upon them are usually unhealthy. These lumps, or tumors, may be caused by defective nutrition, checks, or shakes, or by the depredations of animals or insects.
(G.)Clefts, orsplits, in a tree may be caused by extreme cold, heat, or drouth. They may extend into the tree several inches, and while the blemish will always remain, showing a discoloration or other defect, nature often repairs it so that the strength of the timber is but slightly affected. If these clefts are not healed, rain may find lodgment there, and the sap be so affected that the adjacent wood will be destroyed, and this in time will destroy the tree.
(H.) Eggs of certain insects are also deposited in these clefts, the larvæ of which bore into the wood and destroy it. (I.) If it were not for the birds and other natural enemies of these insects, they would increase so rapidly that the lumber supply would be seriously affected, and, as it is, there are very few trees which are not injured to some extent by wood borers. The United States Bureau of Entomology estimates that the damage to trees by these pests amounts to at least $100,000,000 annually.
(K.) Sometimes (as atd, Fig. 3) branches die or are broken off; this gives the elements access to the heart of the tree and usually causes a permanent injury, as the birds and other denizens of the forest frequently dig out the rotten wood as soon as the weather has had time to do its work, thus giving further opportunity for decay to continue its destructive action. If this does not happen, the wood will grow over the break after several years and leave a loose knot in the heart of the tree, which will be a decided blemish when the log is made into lumber. The sketch shows the endeavor of nature to repair the defect, as the annual rings will eventually close over the break.
The hard knot ate, Fig. 3, is not a serious defect, unless the wood is to be used for finish or where great strength is required.
7. When to cut lumber.—Trees which are intended for the use of wood-workers should not be felled while the sap is in motion. If cut at any other time than midsummer or in winter, the active sap is apt to sour and to cause decay. Owing to the scarcity of lumber, or from avarice, this precaution often is ignored, and trees are felled at any time of year, regardless of their age; in suchcases, much of the lumber lacks the essential qualities of its kind.
When a tree shows signs of dying at the top, it should be cut down, as the quality of the lumber it contains will deteriorate rapidly.
Suggestive Exercises1. What is meant by exogenous trees? In what kinds of wood are the annual layers most prominent? Describe the formation of annual layers. What causes the difference in the degrees of hardness of wood? In the color and odor of wood? How may the age of a tree be determined? Are the broadest annual layers found in young or in old trees? From what class of trees does soft wood come? Hard wood?2. Describe the motion of sap. What forms the sapwood? With what are the cells of the sapwood filled? Compare sapwood and heartwood.3. Describe the four different tissues in a tree. Describe the inner bark.4. What are the medullary rays? In what woods are they most prominent? How do they affect the strength of timber?5. What is the nature of trees which grow in exposed situations? Where are the straightest trees found? Why does the location of a tree affect the grain? What is meant bycoarse,fine,straight, andcross-grainedlumber?6. What causes wind shakes? Heart shakes? Star shakes? How may they be distinguished from each other? What causes discolorations? What is the usual character of timber grown upon marshy ground? What woods are adapted to low ground? What sometimes causes spiral growth? What do lumps and excrescences upon a tree generally signify? What causes clefts in trees? What are the results of clefts? Does nature perfectly repair the cleft? What is the usual result of a branch being broken off?7. What is the per cent of moisture in green wood? Should very young trees be cut? Why? How may the top of a tree show when it should be felled? At what time of year should trees be cut?
Suggestive Exercises
1. What is meant by exogenous trees? In what kinds of wood are the annual layers most prominent? Describe the formation of annual layers. What causes the difference in the degrees of hardness of wood? In the color and odor of wood? How may the age of a tree be determined? Are the broadest annual layers found in young or in old trees? From what class of trees does soft wood come? Hard wood?
2. Describe the motion of sap. What forms the sapwood? With what are the cells of the sapwood filled? Compare sapwood and heartwood.
3. Describe the four different tissues in a tree. Describe the inner bark.
4. What are the medullary rays? In what woods are they most prominent? How do they affect the strength of timber?
5. What is the nature of trees which grow in exposed situations? Where are the straightest trees found? Why does the location of a tree affect the grain? What is meant bycoarse,fine,straight, andcross-grainedlumber?
6. What causes wind shakes? Heart shakes? Star shakes? How may they be distinguished from each other? What causes discolorations? What is the usual character of timber grown upon marshy ground? What woods are adapted to low ground? What sometimes causes spiral growth? What do lumps and excrescences upon a tree generally signify? What causes clefts in trees? What are the results of clefts? Does nature perfectly repair the cleft? What is the usual result of a branch being broken off?
7. What is the per cent of moisture in green wood? Should very young trees be cut? Why? How may the top of a tree show when it should be felled? At what time of year should trees be cut?